Optical recording medium and method of evaluating optical recording medium

ABSTRACT

A conventional mechanical precision measuring device is used effectively in the measurement of the eccentricity of a high density optical disc. A DVR optical disc  2  comprises a pattern  4  of fine depressions and protrusions conformed to the intended format of the disc (a first depressions and protrusions: pits and/or grooves) formed within an information recording area  6  of the disc, as well as a pattern  8  of fine depressions and protrusions for measuring the eccentricity (a second depressions and protrusions), which is formed in an area other than the information recording area  6 , and is formed in accordance with a CD format having lower recording density than that of the format defined for the DVR optical disc  2 . By using this pattern  8  of fine depressions and protrusions for mechanical detection, conventional mechanical precision measuring devices designed for the CD family can be used as it is, for measuring the eccentricity (mechanical precision) of the DVR optical disc  2.

This Application is a 371 of PCT/JP02/10903 filed Oct. 21, 2002.

TECHNICAL FIELD

The present invention relates to an optical recording medium, and moreparticularly to an optical recording medium capable of recording andplayback using an optical system that uses a blue laser, and a method ofevaluating the mechanical precision thereof.

In this description, the term optical recording medium (optical disc)includes not only completed optical recording media (optical discs), butalso semi-completed disc substrates.

BACKGROUND ART

Optical discs such as CD, MD, and DVD comprise an information recordingarea formed from a pattern of fine depressions and protrusions (pitsand/or grooves), which are formed in accordance with a predeterminedformat defined for each disc.

In order to measure the degree of eccentricity relative to the center ofrotation for this optical disc pattern of fine depressions andprotrusions, conventionally a mechanical precision measuring device suchas an eccentricity measuring device or an axial displacement measuringdevice has been used.

A variety of mechanical precision measuring devices have been proposed.

It is a known representative structure in which a laser beam isirradiated through an objective lens that can be driven by an actuator,on the fine depressions and protrusions of the optical disc. In thestructure, the laser beam tracks the depressions and protrusions, sothat the movement (the degree of displacement) of the objective lens,which reflects the displacement in a radial direction (the eccentricity)of the fine depressions and protrusions, is detected as a fluctuation inthe beam spot position on an optical position sensor (for example, seeJapanese Patent Laid-Open Publication No. 1987-117151).

In recent years, optical discs capable of recording and playback usingoptical systems that use blue lasers have been proposed, including theDVR system which uses an optical system with laser light of wavelength405 nm, and a NA value of 0.85.

However, conventional mechanical precision measuring devices used for CDor DVD discs have been produced on the basis of CD or DVD formatproperties (for example, a laser light wavelength λ of 780 nm, anumerical aperture NA=0.45, and a groove pitch of 1.6 μm in the case ofCD, and 650 nm, NA=0.6, and a groove pitch of 0.74 nm in the case ofDVD). Therefore, if the groove pitch of the pattern of fine depressionsand protrusions is reduced to 500 nm or less for example, then thepattern becomes difficult to track so that these conventional devicescannot be adopted as measuring devices for DVR.

Of course, the eccentricity of these types of high density optical discscan be measured by using a DVR (higher performance) mechanical precisionmeasuring device having similar structure to that for CD or DVD.

However, these types of mechanical precision measuring devices aregenerally expensive, even for CD media. Products for DVR media are evenmore expensive. If a new device is required for each new format, thenlarge costs are unavoidable. Moreover, another problem arises in that ifa new measuring device is purchased, the existing mechanical precisionmeasuring devices used for measuring CD or DVD serve as a surplus.

On the other hand, the measurement and evaluation of optical disceccentricity by inspection under a microscope, not by measurement with amechanical precision measuring device, has also been frequentlyperformed. However, even in these microscope measurement methods, whenthe groove pitch of the pattern of fine depressions and protrusions isreduced to 500 nm or less, the required light diffraction phenomenon ishard to be obtained so that the measurement itself becomes moredifficult.

DISCLOSURE OF THE INVENTION

The present invention aims to resolve these conventional problems, withan object of providing an optical disc or an eccentricity measuringsystem, which enables conventional mechanical precision measuringdevices to be effectively used as it is, in the measurement of themechanical precision such as the eccentricity of an optical discconformed to a high density format, and also enables conventionaleccentricity inspection and the like using a microscope.

The present invention is able to achieve the above object by proposingan optical disc according to a construction (1) described below.

(1) An optical recording medium having grooves and/or pits as a firstdepressions and protrusions, of which width is equal to or less than 200nm, in a main information recording area, the optical recording mediumcomprising grooves and/or lands as a second depressions and protrusionsin a predetermined position other than the main information recordingarea, the grooves and/or lands as the second depressions and protrusionsbeing able to be tracked by an optical system in which a laserwavelength thereof is 780±10 nm, and a numerical aperture NA of therecording and playback lens of is 0.45±0.01.

The groove widths listed in this description all refer to half widths.

With the present invention, it is noticed that as long as two formatsdescribed above are either formed consecutively or formed within asingle process (the same disc setting), the precision required inrelation to the optical disc eccentricity (the mechanical precision)does not change significantly, even if the optical disc format ischanged.

Based on this finding, rather than simply purchasing new highperformance mechanical precision measuring devices designed to measurehigher density optical discs, the present invention employs the reversethinking, and adjusts the specifications of the optical disc beingprepared so as to be applied to mechanical precision measuring deviceswhich are conventionally used.

An optical disc according to the present invention comprises a patternof fine depressions and protrusions (pits and/or grooves) conformed tothe intended format of the optical disc, formed within an informationrecording area of the disc, as well as a pattern of fine depressions andprotrusions for mechanical detection, which is formed in an area otherthan the information recording area, and is formed in accordance with aformat with a lower recording density (a wider groove width and a largergroove pitch) than that of the defined format of the optical disc.

As a result, when a high density optical disc such as a so-called DVRdisc is produced, by using this pattern of fine depressions andprotrusions for mechanical detection, conventional mechanical precisionmeasuring devices developed for the CD or DVD families can be usedwithout modification.

In other words, the eccentricity of a wide variety of optical discs withdifferent track shapes (including properties such as the pitch, width ordepth of the pits and/or grooves) can be measured using a singlemechanical precision measuring device.

The pattern of fine depressions and protrusions formed in an area otherthan the information recording area can be inspected by microscope, bymaking the specification into CD level, since the pitch can be set at alevel which enables to recognize a clear light diffraction phenomenon.Accordingly, even in those situations in which even a mechanicalprecision measuring device developed for the CD family of discs cannotbe purchased, if only a microscope is available, eccentricity can stillbe measured in a conventional manner.

The following types of structures could be envisaged as variations ofthe present invention. Details of these variations are given below.

(2) The optical recording medium according to claim 1, wherein a groovepitch of the first depressions and protrusions is equal to or less than500 nm.

(3) The optical recording medium according to claim 1 or 2, wherein agroove width of the second depressions and protrusions is within a rangefrom 400 nm to 600 nm, and a groove pitch is within a range from 1.2 μmto 2.0 μm.

(4) The optical recording medium according to any one of claims 1 to 3,wherein the first depressions and protrusions can be recorded to, and/orplayed back from using an optical system in which a laser wavelengththereof is 450 nm or less, and a numerical aperture NA for the recordingand playback lens is 0.7 or more.

(5) An optical recording medium having grooves and/or pits as a firstdepressions and protrusions, of which width is equal to or less than 200nm, in a main information recording area, the optical recording mediumcomprising grooves and/or lands as a second depressions and protrusionsin a predetermined position other than the main information recordingarea, the grooves and/or lands as the second depressions and protrusionsbeing able to be tracked by an optical system in which a laserwavelength thereof is 640±20 nm, and a numerical aperture NA of therecording and playback lens is 0.6±0.01.

(6) 6. The optical recording medium according to claim 5, wherein agroove pitch of the first section of depressions and protrusions isequal to or less than 500 nm.

(7) The optical recording medium according to claim 5 or 6, wherein agroove width of the second section of depressions and protrusions iswithin a range from 250 nm to 750 nm, and a groove pitch is within arange from 0.6 μm to 1.5 μm.

(8) The optical recording medium according to any one of claims 5 to 7,wherein the first section of depressions and protrusions can be recordedto, and/or played back from using an optical system in which a laserwavelength thereof is 450 nm or less, and a numerical aperture NA forthe recording and playback lens is 0.7 or more.

(9) A method of evaluating an optical recording medium, for evaluatingmechanical precision of grooves and/or pits of a first section ofdepressions and protrusions, which functions as a main informationrecording area and can be recorded to, and/or played back from using anoptical system with a laser wavelength of 450 nm or less, and anumerical aperture NA for the recording and playback lens of 0.7 ormore, wherein a second section of depressions and protrusions, which isprovided in a predetermined position outside the information recordingarea, and can be tracked by any one of an optical system with a laserwavelength of 780±10 nm, and a numerical aperture NA of the recordingand playback lens of 0.45±0.01, and an optical system with a laserwavelength of 640±20 nm, and a numerical aperture NA of the recordingand playback lens of 0.6±0.01, is formed either concurrently orconsecutively with the first section of depressions and protrusions, andthe mechanical precision of the first section of depressions andprotrusions is evaluated by examining the second section of depressionsand protrusions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a schematic illustration of aDVR optical disc used with an optical disc mechanical precisionmeasuring system of the present invention; and

FIG. 2 is a schematic structural diagram of a mechanical precisionmeasuring device used in the above measuring system.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be described indetail with reference to the accompanying drawings.

The optical disc eccentricity measuring system according to thisembodiment applies the present invention to enable the eccentricity(mechanical precision) of a DVR optical disc to be measured using amechanical precision measuring device developed for CD discs.

FIG. 1 shows a schematic illustration of a DVR optical disc substrateused with the present embodiment.

This DVR optical disc 2 comprises an information recording area 6 inwhich a pattern 4 of fine depressions and protrusions (a firstdepressions and protrusions) is formed. In the case of this embodiment,this pattern 4 of fine depressions and protrusions within theinformation recording area 6 is formed only from grooves, with a depthof 20 to 50 nm, a width of 100 to 200 nm, and a radial pitch (groovepitch) of 200 to 400 nm. These values are conformed to the newlyproposed discs known as DVR.

On the other hand, in this embodiment, grooves comprising a pattern 8 offine depressions and protrusions used for measuring the eccentricity (asecond depressions and protrusions) are formed spirally within an innerperipheral area 7A of the area 7 other than the information recordingarea 6. This pattern 8 of fine depressions and protrusions used formeasuring the eccentricity is formed during the same process used forforming the pattern 4 of fine depressions and protrusions of theinformation recording area 6, that is, during a single disc setting.Because the depth of the grooves is dependent on the thickness of thephotoresist, the pattern 8 has a similar depth to that of the groovesdescribed above, whereas the width is from 400 to 1000 nm, and theradial pitch (groove pitch) is from 1.2 to 2.0 μm. These values areconformed to the CD format.

The pattern 8 need not necessarily be formed in a spiral shape, and mayalso be formed concentrically, so that the pattern of fine depressionsand protrusions used for measuring the eccentricity may be formedconcentrically with the information recording area 6. Regardless of theconfiguration adopted, the level of error settles within a range thatcan be effectively ignored. Furthermore, these pattern 8 may also beformed in an outer peripheral area 7B instead of the inner peripheralarea 7A.

FIG. 2 shows a schematic illustration of the structure of a CDmechanical precision measuring device 10 used in the eccentricitymeasuring system according to this embodiment.

This CD mechanical precision measuring device 10 is already known. Inother words, it is one of the important characteristics of the presentembodiment wherein a conventionally known CD mechanical precisionmeasuring device 10 can be used as it is, for measuring the eccentricityof the DVR optical disc 2. The CD mechanical precision measuring device10 is the type of device that is already widely used in the industry,and because the mechanical precision measuring device to be used in thepresent invention is not restricted to a device of this construction,the description below focuses mainly on the functions of the device.

This CD mechanical precision measuring device 10 comprises a laser beamsource 12 for generating a laser beam with a wavelength of 780 nm, inaccordance with the CD format. The laser beam irradiated from this laserbeam source 12 is split into two directions by a spectroscope 14.

One of the split laser beams passes through an objective lens 16, andirradiates the pattern 8 of fine depressions and protrusions used formeasuring the eccentricity, formed on the inner peripheral side 7A ofthe information recording area 6 of the DVR optical disc 2. Thereflected light from the pattern 8 of fine depressions and protrusionsused for measuring the eccentricity is detected by a photodetector 18,and a focus and tracking control circuit 20 then drives an actuator 22based on this detection information. By driving this actuator 22, theobjective lens 16 is moved so that the laser beam tracks the pattern 8of fine depressions and protrusions used for measuring the eccentricity.

The laser beam is irradiated not onto the pattern 4 of fine depressionsand protrusions within the actual information recording area 6, but ontothe pattern 8 of fine depressions and protrusions used for measuring theeccentricity, which is formed on the inner peripheral side 7A of theinformation recording area 6 in accordance with CD format properties.Therefore, no problems arise in tracking the pattern with theperformance provided by the CD mechanical precision measuring device 10.

On the other hand, the other split laser beam is irradiated onto amirror 26 via a fixed lens 24. This mirror 26 is integrated with theobjective lens 16, and follows the radial displacement of the pattern 8of fine depressions and protrusions used for measuring the eccentricity.The reflected beam from the mirror 26 passes through a beam splitter 28and reaches an optical position sensor 30.

As the direction of the reflected beam from the mirror 26 changes, thebeam spot position on this optical position sensor 30 also changes. Thedirection of the reflected beam from the mirror 26 is linked with themovement of the objective lens 16, that is, the radial displacement ofthe pattern 8 of fine depressions and protrusions used for measuring theeccentricity. Namely, the direction of the reflected beam corresponds tothe state of eccentricity for the DVR optical disc 2. Accordingly, bydetecting fluctuations in the beam spot position on the optical positionsensor 30, the state of eccentricity for the DVR optical disc 2 can bedetermined.

The output from the optical position sensor 30 is amplified by anamplifier 32, and output as “eccentricity information”. As a result, thestate of eccentricity of the DVR optical disc 2 can be measured usingthe CD mechanical precision measuring device 10.

EXAMPLE 1

A stamper was prepared. The stamper comprised grooves with a depth of 30nm, a width of 160 nm, and a radial groove pitch of 0.3 μm, inaccordance with the properties of the newly proposed discs known as DVR,which was formed as the pattern of fine depressions and protrusions ofthe information recording area (the first depressions and protrusions).The stamper also comprised grooves with a depth of 30 nm, a width of 500nm, and a radial groove pitch of 1.6 μm, in accordance with theproperties required for the CD format, on the inner peripheral side ofthe information recording area for measuring the eccentricity (thesecond depressions and protrusions).

A two beam cutting machine was used to form the first depressions andprotrusions from the inner periphery to the outer periphery, and then,the cutting machine was returned to the innermost position to form thesecond depressions and protrusions.

The stamper was mounted in a die assembly of an injection moldingmachine in order to mold a resin substrate with an outer diameter of 120mm, and a thickness of 1.2 mm (which belongs to the broad classificationof optical discs according to the present invention).

The resin used was polycarbonate H4000-N282, manufactured by MitsubishiEngineering-Plastics Corporation. The main molding conditions included aresin melt temperature of 360° C., a mold temperature of 125° C., and amold clamping force of 35 tons.

When this resin substrate was measured by using a CD mechanicalprecision measuring device described above (LM1200, manufactured by OnoSokki Co., Ltd., wavelength of laser beam 780 nm, numerical aperture NA0.45), the groove surface formed in the information recording area couldbe focused on, but could not be tracked on the radial direction,resulting measurement of the eccentricity was impossible.

In contrast, as to the grooves formed in accordance with the CD formaton the inner peripheral side of the resin substrate other than theinformation recording area, both focus-on and radial tracking (track-on)were possible, so that eccentricity could be measured with no problems.

From these results, it is evident that by forming a pattern of finedepressions and protrusions conformed to the CD format on an area of aDVR optical disc other than the information recording area, theeccentricity of the DVR optical disc can be measured by using amechanical precision measuring device developed for CD.

EXAMPLE 2

Using the resin substrate of the example 1, the eccentricity thereof wasmeasured by microscope inspection. As a result, the boundary linesbetween the information recording area (the first depressions andprotrusions) and the mirrored area were difficult to be detected by themicroscope, so that measurement of the eccentricity was impossible.However, the boundary lines between the pattern of fine depressions andprotrusions for measuring the eccentricity (the second depressions andprotrusions), formed other than the information recording area and themirrored area were able to be detected even by microscope, so that theeccentricity could be measured easily.

From these results, it is evident that regardless of the format of thepattern of fine depressions and protrusions formed within theinformation recording area, as long as a pattern of fine depressions andprotrusions for measuring the eccentricity having a pitch of the CDformat level is formed on an area other than the information recordingarea, measurement of the eccentricity is also possible using amicroscope in a conventional manner.

Hence it is clear that even for an optical disc in the DVR level (anoptical recording medium in which the width of grooves and/or pits in afirst depressions and protrusions which functions as the maininformation recording area is equal to or less than 200 nm), if either asecond depressions and protrusions of the CD level (a level in which thewidth of the grooves is at least 400 nm but no more than 1000 nm, andthe groove pitch is at least 1.2 μm but no more than 2.0 μm), or asecond depressions and protrusions of the DVD level (a level in whichthe width of the grooves is at least 250 nm but no more than 750 nm, andthe groove pitch is at least 0.6 μm but no more than 1.5 μm), isprovided in a position either on the inner periphery or the outerperiphery of the disc (a predetermined position other than theinformation recording area), then, the mechanical precision of the disccan be satisfactorily measured and evaluated by either a mechanicalprecision measuring device designed for CD or a mechanical precisionmeasuring device designed for DVD.

In other words, it is clear that even though the optical disc itself issurely of the DVR level, with a groove pitch of 500 nm or less, and hasproperties which mean that recording and/or playback can only beconducted with an optical system with a laser wavelength of 450 nm orless, and a numerical aperture NA for the recording and playback lens of0.7 or more, the mechanical precision of the disc can still besatisfactorily measured and evaluated using either a mechanicalprecision measuring device designed for CD or a mechanical precisionmeasuring device designed for DVD.

The CD mechanical precision measuring device 10 described in the aboveembodiment merely represents one possible example, and in the presentinvention, there are no particular restrictions on the construction ofthe mechanical precision measuring device. Specifically, any CD or DVDmechanical precision measuring device that has been proposed, or iscommercially available, can be used, and if new CD or DVD mechanicalprecision measuring devices, such as low cost devices, are developed inthe future, these would also be applicable to the present invention.

INDUSTRIAL APPLICABILITY

According to the present invention, a conventional mechanical precisionmeasuring device can be effectively used, as it is, in the measurementof the mechanical precision of an optical disc with a high densityformat.

1. An optical recording medium including a layer with grooves and/orpits as a first depressions and protrusions, of which width is equal toor less than 200 nm, in a main information recording area; and saidoptical recording medium also including, in the same layer as the firstdepressions and protrusions, grooves and/or lands as a seconddepressions and protrusions in an inner or outer peripheral area withrespect to the main information recording area, the grooves and/or landsas the second depressions and protrusions being able to be tracked by anoptical system in which a laser wavelength thereof is 780±10 nm, and anumerical aperture NA of the recording and playback lens of is0.45±0.01, and wherein the grooves and/or lands as the seconddepressions and protrusions are configured to indicate mechanicalprecision information of the optical recording medium when the opticalrecording medium is evaluated by a mechanical precision measuringdevice.
 2. The optical recording medium according to claim 1, wherein agroove pitch of the first depressions and protrusions is equal to orless than 500 nm.
 3. The optical recording medium according to claim 1,wherein a groove width of the second depressions and protrusions iswithin a range from 400 nm to 600 nm, and a groove pitch is within arange from 1.2 μm to 2.0 μm.
 4. The optical recording medium accordingto claim 1, wherein the first depressions and protrusions can berecorded to, and/or played back from using an optical system in which alaser wavelength thereof is 450 nm or less, and a numerical aperture NAfor the recording and playback lens is 0.7 or more.
 5. An opticalrecording medium including a layer with grooves and/or pits as a firstdepressions and protrusions, of which width is equal to or less than 200nm,in a main information recording area; and said optical recordingmedium also including, in the same layer as the first depressions andprotrusions. grooves and/or lands as a second depressions andprotrusions in an inner or outer peripheral area with respect to themain information recording area, the grooves and/or lands as the seconddepressions and protrusions being able to be tracked by an opticalsystem in which a laser wavelength thereof is 640±20 nm, and a numericalaperture NA of the recording and playback lens is 0.6±0.01, and whereinthe grooves and/or lands as the second depressions and protrusions areconfigured to indicate mechanical precision information of the opticalrecording medium when the optical recording medium is evaluated by amechanical precision measuring device.
 6. The optical recording mediumaccording to claim 5, wherein a groove pitch of the first section ofdepressions and protrusions is equal to or less than 500 nm.
 7. Theoptical recording medium according to claim 5, wherein a groove width ofthe second section of depressions and protrusions is within a range from250 nm to 750 nm, and a groove pitch is within a range from 0.6 μm to1.5 μm.
 8. The optical recording medium according to claim 5, whereinthe first section of depressions and protrusions can be recorded to,and/or played back from using an optical system in which a laserwavelength thereof is 450 nm or less, and a numerical aperture NA forthe recording and playback lens is 0.7 or more.
 9. A method ofevaluating an optical recording medium which includes grooves and/orpits as a first depressions and protrusions formed in an area of a layerof the optical recording medium which functions as a main informationrecording area and can be recorded to, and/or played back from using anoptical system in which a laser wavelength thereof is 450 nm or less,and a numerical aperture NA for the recording and playback lens is 0.7or more, and a second depressions and protrusions formed in an inner orouter peripheral area with respect to the main information recordingarea in the same layer as the first depressions and protrusions, wherethe second depressions and protrusions can be tracked by any one of anoptical system in which a laser wavelength thereof is 780±10 nm, and anumerical aperture NA of the recording and playback lens is 0.45±0.01,and an optical system in which a laser wavelength thereof is 640±20 nm,and a numerical aperture NA of the recording and playback lens of0.6±0.01, is formed either concurrently or consecutively with the firstdepressions and protrusions, wherein the method comprises evaluating themechanical precision of the first depressions and protrusions byexamining the second depressions and protrusions.
 10. The opticalrecording medium according to claim 2, wherein a groove width of thesecond depressions and protrusions is within a range from 400 nm to 600nm, and a groove pitch is within a range from 1.2 μm to 2.0 μm.
 11. Theoptical recording medium according to claim 2, wherein the firstdepressions and protrusions can be recorded to, and/or played back fromusing an optical system in which a laser wavelength thereof is 450 nm orless, and a numerical aperture NA for the recording and playback lens is0.7 or more.
 12. The optical recording medium according to claim 3,wherein the first depressions and protrusions can be recorded to, and/orplayed back from using an optical system in which a laser wavelengththereof is 450 nm or less, and a numerical aperture NA for the recordingand playback lens is 0.7 or more.
 13. The optical recording mediumaccording to claim 6, wherein a groove width of the second section ofdepressions and protrusions is within a range from 250 nm to 750 nm, anda groove pitch is within a range from 0.6 μm to 1.5 μm.
 14. The opticalrecording medium according to claim 6, wherein the first section ofdepressions and protrusions can be recorded to, and/or played back fromusing an optical system in which a laser wavelength thereof is 450 nm orless, and a numerical aperture NA for the recording and playback lens is0.7 or more.
 15. The optical recording medium according to claim 7,wherein the first section of depressions and protrusions can be recordedto, and/or played back from using an optical system in which a laserwavelength thereof is 450 nm or less, and a numerical aperture NA forthe recording and playback lens is 0.7 or more.
 16. The opticalrecording medium according to claim 1, wherein the mechanical precisioninformation includes a degree of eccentricity of at least the firstdepressions and protrusions.
 17. The optical recording medium accordingto claim 5, wherein the mechanical precision information includes adegree of eccentricity of at least the first depressions andprotrusions.